This invention relates to the removal of carbonyl sulfide (COS) from liquid petroleum hydrocarbons by catalytic hydrolysis over alumina.
Carbonyl sulfide is undesirable in petroleum hydrocarbons because it is a sulfur source, and therefore a potential atmospheric pollutant and contaminant of industrial processes. A significant instance of the latter is the poisoning of polymerization catalysts by the COS commonly present in petroleum-derived polymerizable olefins such as propylene. With the advent of high activity catalysts, however, the COS must be reduced to levels well below those previously required. Thus, the COS level may be required to be reduced to below 1 ppm (parts per million by weight) and sometimes to levels below 100 ppb (parts per billion by weight) e.g., to 50 ppb or less.
The ability to attain the lower levels of COS must also be coupled with high reaction rate and capability of practice on an economic scale. For reasons discussed subsequently, high reaction rates and the smaller reactor sizes which can be used as a consequence of higher reaction rates, are of particular importance in the removal of COS from liquid petroleum hydrocarbons by catalytic hydrolysis over alumina. From the standpoints of process efficiency and economy, treater sizes (gross alumina bed volumes) of 800 cu. ft. or less, preferably 600 cu. ft. or less, are desirable.
U.S. Pat. No. 3,265,757 to Frevel and Kressley describes the hydrolysis of COS in liquid aliphatic hydrocarbons by treatment at 20.degree. C. to 50.degree. C. with an alumina catalyst wherein an amount of water equal to 0.3A ppm to 12T ppm (where A is the COS concentration in ppm (wt.) and T is degrees centigrade temperature) is maintained in the hydrocarbons during the treatment. The only treatment conditions specifically mentioned are treatment at 32.degree. C. of liquid propylene containing 49 ppm COS at a propylene flow rate of 60 gallons per minute through an alumina bed, while feeding ion-free water to the propylene feed stream at 0.0104 gallons per minute to thereby maintain 290 ppm of water in the propylene. (The 290 ppm of water provides 19.7 times the stoichiometric amount of water required for reaction with COS.) The COS was reduced to less than 1 ppm (detection limits of the analysis). A predecessor Frevel and Kressley patent, U.S. Pat. No. 3,058,800, describes catalytic hydrolysis of COS in a gaseous stream over alumina at 35.degree. C. to 250.degree. C. wherein from 1 to about 2 moles of water per mole of COS preferably is maintained in the gas stream. Neither patent provides any specific guidance to process results, and reaction rates specifically, when treating liquid petroleum hydrocarbons at less than the 290 ppm of water recited in Example 1 of U.S. Pat. No. 3,265,757.
British patent publication No. 2,108,146 published May 11, 1983 citing U.S. application Ser. No. 298,702 filed Sept. 1, 1981 for priority, describes the catalytic hydrolysis over a platinum sulfide/alumina catalyst of COS in gaseous or liquid propylene, containing at least double the stoichiometric amount of water required for reaction with the COS. As compared to the use of alumina alone as the catalyst, the process is burdened by the cost of the platinum sulfide catalyst and the necessity of regenerating the platinum sulfide.
Alumina is disclosed as a dehydrating agent for gases containing COS in U.S. Pat. No. 3,000,988 to Karchmer and Walker. U.S. Pat. No. 2,772,208 to Ferm incidentally discloses that alumina dehydrates liquefied petroleum gas while also decomposing COS in the liquefied gas to form corrosive products. Neither patent discloses or suggests that water content in the gas might be critical for effective catalytic decomposition of the COS over alumina alone.